Hydrofluorocarbons are useful as an etching gas for the microfabrication of semiconductors, liquid crystals, etc. In particular, methane fluoride (CH3F) has been attracting attention as an etching gas for forming state-of-the-art micro-structures.
For example, the following methods are known as methods for producing methane fluoride:
(1) a method comprising reacting methyl alcohol and hydrogen fluoride using a catalyst (PTL 1);
(2) a method comprising reacting methyl chloride and hydrogen fluoride using a catalyst (PTL 2);
(3) a method comprising pyrolyzing 1-methoxy-1,1,2,2-tetrafluoroethane (PTL 3); and
(4) a method comprising reacting dimethyl sulfate with an alkali metal fluoride, such as potassium fluoride, in the presence of a solvent having a polarity, such as diglyme or sulfolane, to produce monofluoromethane (PTL 4).
Among these methods, method (1) has the following disadvantages. The catalyst easily deteriorates due to the large amount of water generated. Manufacturing facilities easily corrode due to hydrofluoric acid produced as a result of the dissolution of unreacted hydrogen fluoride in the generated water.
Method (2) has the following problems. Because an excess of hydrogen fluoride must be added to improve the fluorination reactivity, the recycling and reuse of the hydrogen fluoride require large facilities and greatly increase the cost of manufacturing facilities. Furthermore, the introduction of moisture or the like as a contaminant may lower the reactivity and corrode the manufacturing facilities.
Further, method (3) requires cooling energy to separate methane fluoride (boiling point: −79° C.) from difluoroacetic acid fluoride, which is produced simultaneously with methane fluoride and which has a boiling point as low as 0° C. Furthermore, the resulting mixture contains a large amount of impurities, and separation of the impurities from methane fluoride is difficult even by rectification. In particular, among the impurities, trifluoromethane (CHF3) has a boiling point of −84° C., which is close to the boiling point of methane fluoride, and is thus difficult to separate from methane fluoride. Furthermore, since the amount of trifluoromethane produced is associated with the starting material conversion, reducing the reaction conversion may be necessary to reduce the amount of trifluoromethane, which reduces production efficiency. Because the starting material 1-methoxy-1,1,2,2-tetrafluoroethane is synthesized by reacting tetrafluoroethylene and methanol, a risk of handling tetrafluoroethylene is also involved, and high costs of the starting materials and manufacturing facilities become a problem.
Lastly, method (4) requires a high-temperature reaction (about 150° C.) to obtain a sufficient amount of the product.